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We investigate the interactions between natural resource-based poverty traps and food security for smallholder farms in highland Kenya using a recently developed system dynamics bio-economic model. This approach permits examination of the complex interactions and feedback between farm-household economic decision-making and long term soil fertility dynamics that appear to characterize persistent poverty and food insecurity among smallholders in rural highland Kenya. We examine the effects of changing initial endowments of land and stocks of on-farm soil organic matter on smallholders’ well-being, as reflected in several different indicators. We show that larger and higher quality land endowments permit accumulation of cash and livestock resources and conservation of soil organic matter relative to smaller or more degraded farms. This suggests the existence of asset thresholds that divide food secure households from insecure ones.

The soil environment is a primary component of the global biogeochemical sulfur (S) cycle, acting as a source and sink of various S species and mediating oxidation state changes. However, ecological significance of the various S forms and the impacts of human intervention and climate on the amount and structural composition of these compounds are still poorly understood. We investigated the long-term influences of anthropogenically mediated transitions from natural to managed ecosystems on molecular-level speciation, biogeochemical dynamics, and the apparent temperature sensitivity of S moieties in temperate, subtropical, and tropical environments with mean annual temperature (MAT) ranging from 5 degrees C to 21 degrees C, using elemental analysis and X-ray absorption near-edge structure (XANES) spectroscopy. Land-use and land-cover changes led to the depletion of total soil S in all three ecoregions over a period of up to 103 years. The largest decline occurred from tropical forest agroecosystems (67% Kakamega and 76% Nandi, Kenya), compared to losses from temperate (36% at Lethbridge, Canada, and 40% at Pendleton, USA) and subtropical (48% at South Africa) grassland agroecosystems. The total S losses correlated significantly with MAT. Anthropogenic interventions profoundly altered the molecular-level composition and resulted in an apparent shift in oxidation states of organic S from native ecosystems composed primarily of S moieties in intermediate and highly reduced oxidation states toward managed agroecosystems dominated by organic S rich in strongly oxidized functionalities. The most prominent change occurred in thiols and sulfides, the proportion of which decreased by 46% (Lethbridge) and 57% (Pendleton) in temperate agroecosystems, by 46% in subtropical agroecosystems, and by 79% (Nandi) and 81% (Kakamega) in tropical agroecosystems. The proportion of organic S directly linked to O increased by 81%, 168%, 40%, 92%, and 85%, respectively. Among the various organic S functionalities, thiols and sulfides seem to have higher apparent temperature sensitivity, and thus these organic S moieties may become prone to losses due to land-use changes, even from the cooler regions of the world if MAT of these regions rise in the future.

Raising agricultural productivity in smallholder agriculture in sub-Saharan Africa requires an understanding of if and how farm household land use and socioeconomic factors affect soil fertility. Market access, population growth, socio economic characteristics and agro ecological zones have been proposed as important drivers of land use intensity and, consequently, soil fertility. We used diffuse reflectance infrared spectroscopy to measure soil fertility, and multivariate and exogenous switching regression statistical approaches to determine if soil fertility in the smallholder farms of the highlands of Kenya is associated by region, land use categories (cash crop, food crop, fodder and pastures), and selected household socio economic factors (household income, number of adults, farm size and number of cattle). Over 2000 fields on 236 farms were sampled in Embu (eastern Kenya highlands, primarily Andosols) and Madzuu (western Kenya highlands, primarily Ferrasols). Soil fertility variables, including total soil carbon (SC), total nitrogen (TN), pH, available Olsen phosphorous (P), extractable potassium(K), calcium (Ca), and magnesium (Mg), effective cation exchange capacity (ECEC) and texture, were measured using conventional laboratory techniques on 15% of the sampled soils. From these analyses, SC, TN, P and K were all greater in Embu compared to Madzuu soils. Soil fertility variables were significantly higher in pastures compared to other land uses in Madzuu, but were comparable with other land uses in Embu. This soil data was then used to calibrate soil reflectance results in order to predict soil fertility variables for all soil samples. Principle component analysis (PCA) of soil fertility variables developed from the spectroscopy data for each soil sample indicated similarities among sites in the three most important eigenvectors: the first (soil nutrient) vector had high positive loadings for K, Ca, Mg, ECEC and pH; the second (soil organic matter, SOM) vector had high positive loadings for soil organic carbon and total nitrogen; and the third (soil texture) vector had high positive loadings for clay plus silt. However, in Embu, P was associated with the soil organic matter vector while in Madzuu it was associated with the soil nutrient vector. In comparison to pasture all other land uses were associated with lower values of soil nutrient and SOM components in Madzuu, while in Embu, these other land uses were associated with higher values of the SOM component. Number of cattle per farm had no association with any of the three soil fertility components at either site. In Embu, farm income and adult population were both positively related to SOM. In Madzuu, farm size was positively associated with SOM but negatively associated with soil nutrients. More than twice as much P fertilizer is applied on average in Embu compared to Madzuu (27 vs. 11 kg ha-1 season-1). Our study supports the link between poverty dynamics and soil degradation in smallholder agriculture; wealthier households in the eastern Kenya highlands are able to invest in soil fertility management while the poorer households in western Kenya are mining nutrients in soils.

That farmers rely on the land for their livelihoods is obvious. The converse, that ecosystem services depend on farmers' behaviors, must also be recognized if agricultural productivity is to be improved. In sub Saharan Africa, the 70% of the population employed in the agricultural sector (Sanchez 2002) is engaged in an on-going 'dialogue' with the agricultural natural resource base. Recently, this conversation has not been going well: per capita food production has remained stagnant for the last 40 years so now 180 million on the continent lack adequate food, a number that has increased by 100% since 1970 (Sanchez 2002). To provide adequate diets to the African population, increases in crop yields of 3.0 to 3.5% y-1 are needed (Reardon et al. 2001), but such increases have not been realized as average maize yields have remained static at 1200 kg ha-1.